1. Field of the Invention
The present invention relates to a zero phase voltage measuring device for use in three phase power system and, more particularly, to a zero phase voltage measuring device for measuring a zero phase voltage of the three phase power system having an earth-fault.
2. Description of the Prior Art
FIG. 7 shows an example of conventional zero phase voltage measuring device Zc applied to a three phase power system having a first power line La of phase A, a second power line Lb of phase B, and a third power line Lc of phase C.
The zero phase voltage measuring device Zc includes a first voltage sensor 51 which is provided at the first power line La to detect the line-to-ground voltage thereat and produce a first voltage signal Sa in proportion to the detected voltage. A second and a third voltage sensors 52 and 53 are also provided for detecting and producing a second and a third voltage signals Sb and Sc in proportion to the line-to-ground voltages at lines Lb and Lc, respectively, in a manner similar to that of the first voltage sensor 51. As these voltage sensors 51, 52, and 53, a non contact type sensor such as an optical voltage sensor is used.
The zero phase voltage measuring device Zc further, includes a signal processing unit 54 which adds the voltage signals Sa, Sb, and Sc to produce a zero voltage signal which is in proportion to the zero phase voltage of the three phase power system.
However, the conventional zero phase voltage measuring device Zc requires three of the optical voltage sensors 51, 52, and 53 which are very expensive, resulting in such disadvantages that the cost is increased and the handling of three voltage sensors to set the power lines is very complicated at the measuring site. Furthermore, it is necessary to make a correction on each of three voltage sensors, which is a difficult and bothersome task.